1. Field of the Invention
The present invention relates to a slave driving force-transmitting mechanism for a vehicle, for constructing a power-transmitting system disposed on a side of a slave driving shaft of the vehicle provided with a driving mechanism.
2. Description of the Related Art
A system has been known for four-wheeled vehicles, in which one of the front and rear axles is directly connected to an engine, and the driving torque (hereinafter simply referred to as "torque") is transmitted from the one axle (master driving shaft) to the other axle (slave driving shaft) through a viscous fluid joint which acts in response to the relative velocity of rotation (see Japanese Laid-Open Patent Publication No. 58-50349).
The viscous fluid joint has such a characteristic that the torque to be transmitted changes depending on the difference in velocity of rotation between the master and slave driving shafts. If the difference in velocity of rotation between the master and slave driving shafts exceeds a certain limit, there is given a state in which the master and slave driving shafts are operated as if the both are substantially directly connected. Therefore, it is necessary to set such that the master and slave driving shafts have an equivalent strength to resist to the torque. On the other hand, in the case wherein the difference in velocity of rotation between the master and slave driving shafts is extremely small, the torque transmitted from the master driving shaft to the slave driving shaft is substantially equal to zero. Therefore, the load imposed on the slave driving shaft is extremely relieved in this state.
The system described above for transmitting the torque from the master driving shaft to the slave driving shaft by using the viscous fluid joint includes no function to prescribe the upper limit of the transmitted torque. Therefore, the following inconvenience may occur. Namely, large torque is imposed on joint mechanisms which are provided for a driving shaft (drive shaft) and a power-transmitting shaft (propeller shaft) disposed on the side of the slave driving shaft. As a result, vibration occurs due to friction between constitutional components of the joint mechanisms, and the vibration is transmitted to the vehicle's body. Further, an additional inconvenience may occur. Namely, the joint mechanisms suffer seizure due to frictional heat.
Further, the vehicle is sometimes forced to continue running in a state in which front and rear wheels substantially have different effective diameters as a result of installation of a spare tire or installation of a chain to prevent slipping during running on a road during snowy conditions. In such a state, a difference in the velocity of rotation occurs between the front and rear wheels, and unnecessarily large torque is transmitted to the slave driving shaft, although the running state does not primarily require transmission of torque between both the master and slave driving shafts.
Namely, it is sufficient, for the transmission of torque between the master and slave driving shafts during ordinary operation, to consider only a transition state, while the torque is continuously transmitted to the slave driving shaft when the front and rear wheels substantially have different tire diameters as described above. In general, the vibration, which is caused by friction in the joint mechanism used for the power-transmitting system, is occasionally transmitted to the vehicle's body even in the case as described above.
Therefore, the conventional technique adopts a friction-reducing mechanism such as a needle bearing provided at sliding sections of components for constructing the joint mechanism, in order to avoid friction and seizure which would be caused by frictional heat generated in the joint mechanism.
The present invention proposes a technical concept in which the substantial load imposed on the side of a slave driving shaft is relieved to give relief to an entire driving mechanism by regulating the upper limit of torque transmitted from a master driving shaft to a slave driving shaft by using a transmission torque-restricting means (see Japanese Laid-Open Patent Publication No. 63-49526). The present invention proposes a technical concept in which the upper limit of torque transmitted to a slave driving shaft through a transmission torque-restricting means is regulated, and the transmitted torque is decreased in response to the increase in velocity of a vehicle (see Japanese Laid-Open Patent Publication No. 3-224831).